Two other planes, the A-12 and the YF-12, could easiy be mistaken for the SR-71. The A-12 was the first plane developed out of the three. It is actually a host plane for the smaller, faster, and higher-flying D-21 drone, code-named Tagboard, which sat piggyback on the A-12 and used a ramjet engine once released for flight. The project was soon cancelled, however, due to a fatal accident, and the D-21 went on to use the B-52 as a transport host. The YF-12 was an SR-71 with an internal bay carrying three Hughes GAR-9/ AIM-47A air to air radar guided missiles, designed to shoot enemy airplanes flying at lower altitudes. Only three YF-12s were ever built.

Summary of YF-12 test missile firings

Test date
Speed of YF12
Altitude of YF12
Altitude of target
March 1965
2.19 M
65000 ft
40000 ft
destroyed
May 1965
2.18 M
64800 ft
20000 ft
missile gyro failure
September 1965
3.22 M
75200 ft
20000 ft
destroyed
March 1966
3.16 M
74000 ft
1700 ft
destroyed
April 1966
3.20 M
75200 ft
1700 ft
destroyed
May 1966
3.20 M
76000 ft
20 000 ft
destroyed
September 1966
3.20 M
74400 ft
500 ft
destroyed

A-12, D-21, Specifications

D-21A/B
A-12
Construction: Titanium (Beta-120/Ti-13V-11Cr-3A1)
Monococque with some super-high-temperature plastics.		 Titanium (Beta-120/Ti-13V-11Cr-3A1)
Monococque with some super-high-temperature plastics.
Length: 42 ft 10 in 102 ft 3 in
Height: 7 ft 1/4 in 18 ft 6 in
Wingspan: 19 ft 1/4 in 55 ft 7 in
Wing Area:  1,795 square feet
Maximum take-off weight: 11,000 pounds 117,000 pounds
Maximum Speed: 3.35 M 3.2 M
Armament: - -
Ceiling: 95,000 ft 85,000 ft
Powerplant: Marquardt RJ43-MA-11 Ramjet with 1,500 pounds thrust 2x Pratt & Whitney J75 with 17,000 pounds thrust each (test flights)
2x Pratt & Whitney J58 (JT11D-20A) high-bypass ratio turbojets with 20,500 pounds thrust each
Production: 38 15

D-21


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